The intimate anatomical and biochemical relationship between the caudate nucleus and the substantia nigra suggests that they may subsume similar cognitive functions. It has been established that lesions placed in the caudate nucleus of the rhesus monkey impair performance on delayed alternation performance. However, massive lesions placed in the substantia nigra of a pilot animal had little, if any, effect. It becomes important therefore to establish the generality of this negative finding, and to explore what significance for behavior there is in the biochemical and anatomical similarities in these structures.